This invention relates generally to switching power supplies which use a semiconductor device, such as a transistor, to selectively permit current flow through an inductor. Specifically, this invention relates to circuits which drive a control node, such as a base node of the transistor, in response to a pulsewidth modulated control signal.
Semiconductor devices used in switching power supplies operate in a precisely controlled manner to insure efficient and reliable operation of the semiconductor devices. For example, efficient and reliable operation of a switching transistor requires driving a base node of the transistor so that three goals are achieved. For the first goal, a relatively high, peak current pulse is supplied to the transistor base at the instant the transistor activates. This activating peak current pulse permits the transistor to experience a relatively fast activation time. As a result, the transistor quickly achieves a saturation state and dissipates a relatively small amount of power in the process of transitioning from an inactivated to an activated state.
A second goal concerns providing only sufficient steady state current to the transistor base to keep the transistor in saturation. This steady state base current should be less than the activating peak current pulse and should not increase to a destructive value even under worst case conditions.
The third goal concerns inactivating the transistor. Accordingly, a base-emitter voltage of the transistor must reverse polarity so that the transistor will experience a relatively fast inactivation time. The fast inactivation time permits the transistor to transition between an active state and an inactive state rapidly and without dissipating a great amount of power.
By achieving the three above-mentioned goals, a switching transistor may operate in a relatively efficient manner. The switching device may also operate in a relatively reliable manner because it dissipates a minimal amount of power. Although circuits which achieve one or more of the above goals are known in the art, such circuits tend to ignore the efficient and reliable operation of the circuit which causes the switching device to achieve these three goals. For example, many of these circuits drive a transistor base node from a high voltage source. Although a high voltage source may be needed to achieve the activating peak current pulse, it is typically not needed to supply the steady state current. Accordingly, the steady state current experiences a relatively high voltage drop, and causes relatively great power consumption.
Some devices directly couple the base drive circuit to an unregulated line voltage. The unregulated line voltage exhibits relatively unpredictable characteristics. Thus, worst case base drive current is difficult to control, and a relatively unreliable circuit results. Other base drive circuits require additional regulated voltage sources for reliable operation. However, the additional regulated voltage sources increase complexity, lead to an increased power dissipation, and degrade overall reliability.